Welding robot systems having spot welding guns are described, for example, in U.S. Pat. Appl. Pub. No. 2011/0089146 to Takahashi et al. and U.S. Pat. No. 5,898,285, to Nagasawa et al. A typical spot welding servo gun includes a main body having a stationary electrode tip and a movable electrode tip. The stationary electrode tip is disposed opposite the movable electrode tip. The stationary electrode tip is generally immovable relative to the main body of the spot welding gun, and the movable electrode tip is mounted on the main body and opened and closed during a welding operation.
Both electrode tips must press a part with equal force, for good weld quality If the tips do not touch the part, re-teaching or teach correction is required. However, the teach correction of the welding robot is time consuming.
The known automatic teach correction methods do not determine whether the gun electrodes are normal to a surface of the part being welded, so orientation of the spot welding gun is not corrected. Orientation error, where the spot welding servo gun is not normal to a part to be welded, undesirably results in uneven force distribution, uneven current density over the spot, and, ultimately, poor weld quality. Orientation error can be difficult and time consuming to correct, especially when one of the gun electrodes is not completely visible.
There is a continuing need for a method for correcting an orientation of a spot welding gun, including determining a normal vector relative to a surface of a part to be welded. Desirably, this information is used to correct the orientation of the spot welding gun prior to a welding operation.